The present invention relates to ultrasound transmission and reception utilizing, for example, ultrasound measurement and imaging in the field of medicine and the field of ultrasound measurement. In particular, it is related to ultrasound transmission and reception using pulse compression.
Conventionally, measurement has been performed and images obtained utilizing reflected ultrasound waves and the like. For example, with an ultrasound diagnosis apparatus, a tomogram of an organism is obtained by transmitting an impulse wave from an ultrasonic transducer, receiving back the reflected echo, and then being subjected to image processing. For such an ultrasound diagnosis apparatus, the deepest invasion depth and highest resolution possible is required.
There is a pulse compression technique that satisfies this requirement. With this, an originally long pulse is shortened and compressed by subjecting an ultrasonic signal that is to be transmitted to FM modulation (hereafter the resulting signal is referred to as a chirp signal) and upon reception, passing it through a filter corresponding to the chirp signal. An attempt is made to increase resolution due to the compression, and at the same time improve the signal-to-noise ratio, and improve the invasion depth.
With such pulse compression, since the transmitted signal and the received signal are temporally separate, it is necessary to separate the transducer from measurement subject. The region in between is called the separation region. For example, with an ultrasound microscope, the separation region is configured by a line that has a sufficiently large diameter in comparison to the signal wavelength and that is used as a delay medium. This line cannot be regarded as a waveguide because it may not be flexible due to the fact that it is allowed to have an infinitely large diameter. Here the waveguide is said to have no change in amplitude distribution within cross sections within the entire propagated distance. In this case, transmission/reception of a pulse having a long duration of 100 xcexcs or longer using the 20 Mhz band is practically difficult. In addition, since this line has no flexibility, it cannot be adapted for ultrasound endoscopes, etc. As a method in place of this, there is a method that uses separate probes for transmission and reception. In addition, a circulator is used with a 25 Mhz or higher band. However, a reflection occurring due to mismatch between the transducer and the transmission medium is additionally mixed into the reception system.
Pulse compression is widely used with the objective of attempting to increase transmission energy under the limitation of transmission peak-power in the field of radar and sonar in order to increase survey distance and/or gain higher resolution. Much research on introducing the pulse compression technique with similar objectives is also being carried out in the field of medical ultrasound. Notwithstanding benefits such as being able to improve resolution in a predetermined region in order to allow manipulation of the transmission signal spectrum in the time domain, this pulse compression technique has yet to reach realization in the field of medical ultrasound.
The biggest hurdle to bringing this into use is the need for a separation region, and the next hurdle is suppression of the side lobes after pulse compression. The problem with the latter is that due to the signal of a small reflective body becoming buried by the side lobes of a signal from a large reflective body.
Speaking of the separation region problem, with the pulse compression technique, the long pulse width of the transmitted pulse signal, which is hundreds of micro seconds long, causes separation to become great. A soft plastic board is ordinarily used to provide this region. In actuality, this method is very difficult to deal with. In addition, this method is not applicable to ultrasound endoscopes, etc. Another way of avoiding this method is using separate proves for transmission and reception. However when separate probes for transmission and reception are used, only a signal from the region where the transmitted ultrasound beam intersects the region where the reception transducer can receive, and the obtained image is poor. Furthermore, for reception of a mixed transmission signal and reception signal it is not practiceal because it requires an amplifier with extremely large dynamic range. Accordingly, a method of separating a transmission signal and a reception signal with an integrated transmission/reception probe is desirable.
The object of the present invention is to solve the following problems occurring in conventional ultrasound transmission and reception.
1. In pulse compression, a transmission signal and a reception signal having long duration cannot be temporally separated by a single transducer.
2. After compression, side lobe level suppression is insufficient.
Once these methods are developed, various applications such as detection of an extremely weak signal and Doppler measurement will become possible.
In order to achieve the above-mentioned object, an ultrasound transmission/reception apparatus, which performs pulse compression on a received ultrasound signal using a signal with temporally changing frequency as an ultrasound signal to be transmitted, is characterized by a transducer common for receiving and transmitting said ultrasound signal and a transmission line common for propagating said ultrasound signal, wherein a flexible waveguide transmission line is used as said transmission path, and said transmission line is used as a delay medium to temporally separate a received ultrasound signal and a transmitted ultrasound signal with long duration. A quartz rod where the center portion is made narrow can be used as this transmission line.
For said ultrasound signal to be transmitted, a signal where frequency changes but not dependent on time may be used. In this case, said signal to be transmitted may be a signal that becomes a signal where frequency changes in proportion to time when received. When the transmission line is long, a chirp signal with a changing frequency becomes distorted; however, by using a non-linear chirp signal where frequency changes but not in proportion to time, it becomes possible to reduce the distortion in the received signal.
Side lobe suppression can be performed after pulse compression of a received ultrasound signal, by taking the correlation with an ideal compression waveform during further compression.
By coding a plurality of ultrasound signals delayed a certain length of time in conformity with whether or not being sent according to a code series, and transmitting them, and after pulse compressing the received signals, they may be decoded in conformity with the code series that has been coded. In this manner, by transmitting a plurality of signals in accordance with the code series, two-step compression processing becomes possible, and a reception signal with an even higher SIN ratio can be obtained.
By using an up chirp signal and a down chirp signal as the ultrasound signal to be transmitted, and performing analysis of the time difference or spectrum of, for example, the compressed pulses obtained through processing said respective signals received, it is possible to accurately measure the Doppler effect.
In addition, the above mentioned transmission/reception configuration can be applied to configure an interluminary system.